Increasing the capacity of a cellular radio network

ABSTRACT

A cellular radio network and a method for increasing traffic carrying capacity in a cellular network in which the operating frequency spectrum of the cellular network has been divided so that typically both regular frequencies and super-reuse frequencies are employed in each cell. The regular frequencies use a conventional frequency reuse pattern to provide seamless overall coverage (overlay). A very tight frequency reuse pattern is used for the super-reuse frequencies to provide additional capacity (underlay). The cellular network controls the division of traffic into regular and super-reuse frequencies by radio resource allocation at the call set-up phase and later on during the call by handover procedure. The cellular network continuously monitors the downlink co-channel interference of each super-reuse frequency in the cell separately for each ongoing call. The call is handed over from a regular frequency to a super-reuse frequency when the co-channel interference level on the super-reuse frequency is sufficiently low. When the co-channel interference level on the super-reuse frequency deteriorates, the call is handed over from the super-reuse frequency back to the regular frequency.

This application is the national phase of international applicationPCT/FI96/00540, filed Oct. 11, 1996 which designated the U.S.

FIELD OF THE INVENTION

The present invention relates to cellular radio networks andparticularly to methods for increasing the capacity of a cellular radionetwork.

BACKGROUND OF THE INVENTION

The most significant factor reducing the capacity of radio systems isthe limited frequency spectrum available. The capacity of a radio systemis thus dependent on how efficiently the radio frequencies allocated tothe system can be utilized. In cellular radio networks, enhancedutilization of radio frequencies is based on frequency reuse; the samefrequency is reused at several locations that are sufficiently spacedapart, which affords a vast increase in system capacity. This iscounteracted by increased complexity in the network as well as in themobile units which must be capable of selecting a base station fromamong several possible base stations. For example, if the same frequencyis reused in every ninth cell, the spectral allocation of N frequenciespermits the use of N/9 carriers simultaneously in any cell. Diminishingthe cell size or reducing the distance between cells using the samefrequency will enhance capacity on the one hand, but also increasesco-channel interference. Therefore, selection of the reuse factor isoften a compromise between co-channel interference and the trafficcarrying capacity of the system.

Since the frequency spectrum allocated to a cellular radio network isfixed and the number of subscribers is rapidly increasing, efficient useof the allocated frequency spectrum is vital to any network operator.Hence, various features increasing the traffic carrying capacity in thecellular network will provide much-needed relief to operators,particularly in crowded urban areas. Radio network evolution towardshigh-capacity radio networks has the following main alternatives:increasing the number of channels, splitting the cells (small cells),microcellular networks, multi-layered networks, underlay-overlaynetworks, and other capacity enhancement concepts, such as half-ratechannels, frequency hopping, and power control. In the following, thesealternatives will be described in more detail.

Increasing the Number of Channels

The simplest way to supplement capacity is by increasing the number ofchannels. Since the allocated cellular spectrum per network operator isvery limited, this method does not give relief from capacity problems.

Splitting Cells (Small Cells)

When cell sizes are reduced below a radius of 1 km, there generally is aneed to lower the antenna height below rooftop level. This is becausecoverage to localized areas at street level cannot be efficientlyengineered from a rooftop installation. Such lowering of antennas causesproblems in designing coverage. Prediction of ranges for these types ofinstallations is less well understood than in cases of macrocells.Furthermore, interference management becomes more difficult from belowrooftop installations, as overspill into co-channel base stations cannotbe equally controlled. Cell overspill may eventually reduce cell sizesto the point where conventional planning practices and radio systems donot work efficiently. Additionally, any significant capacity enhancementis accompanied by major in vestments in BTS sites and transmissionconnections. Splitting of cells is a good method for capacity relief upto a certain point. Unfortunately, urban area capacity requirements areso high that this method does not offer help in the long run. Cellsplitting can therefore only be used for short term relief.

Microcellular Network

There is no exact definition of "microcellular network". A cell having asmall coverage area and antennas below rooftop level could be thecharacteristics in the definition of a "microcell". Microcellularconcepts are often mistakenly referred to as "multi-tiered", but a"microcell" can be deployed without a multi-layer architecture. Inimplementing cell splitting below a certain limit and placing antennasbelow rooftop level or in buildings, advanced solution radio networkplanning and radio resource control is needed. An increased number ofBTS sites significantly increases the costs. For cells with a radius of300 m-1 km, signal variability due to shadow fading is very highcompared to macrocells and relative to the coverage area of the smallcells. These factors mean that cell overlaps need to be very high inorder to meet the desired overall coverage; this is, of course,inefficient. Cells with a radius below 300 m experience more line ofsight signal propagation and somewhat less signal variability, which ishelpful from a coverage point of view. However, antenna location inthese circumstances very significantly determines the actual cover agearea. Localized blockages which cause serious shadows effectivelyproduce coverage holes. Small antenna location variations significantlyalter the effectiveness and characteristics of the BTS site. There aretwo alternative solutions to these problems: to deploy more cellsaccepting the inefficiency of high cell overlap, or significantlyincrease and improve engineering effort in the actual BTS site selectionand planning process. Both of these solutions increase the costs to theoperator. The net result is that a microcellular network does not give asignificant capacity increase without major investment in BTS sites andtransmission connections.

Underlay-Overlay Network

To cope with the two conflicting goals in radio network design, i.e.,coverage and capacity, it is possible to build a radio network which hastwo (or more) separate cell layers, one, e.g., a macrocell layer,providing overall coverage and the other, e.g., a microcell layer,providing capacity. The "coverage layer" uses a conventional frequencyreuse pattern and cell range to provide seamless overall coverage. The"capacity layer" uses a very tight frequency reuse pattern and a shortercell range to achieve high capacity with a few channels. Multi-layerednetworks are often also referred to as "underlay-overlay" networks.

In an underlay-overlay network, there are many ways to control thehandover between layers. The handover decision can be made on the basisof field strength or power budget values. In this case, the interferencelevel must be predefined for each BTS site and the handover thresholdsand transmit power are adjusted to minimise the interference. Theinterference control is always a statistical method and the resultingaverage quality is therefore not a quality guarantee for a singleconnection. For this reason, the achieved increase in capacity isquestionable.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve frequencyutilization in an underlay-overlay cellular radio network withoutincrease in co-channel interference, and thereby to significantlyimprove capacity without any major additional investments or extensivemodifications to the network.

This and other objects of the invention will be achieved with a cellularradio network in which allocated radio frequencies are divided intoregular radio frequencies for which lower frequency reuse is utilized toachieve a seamless overall coverage, and super-reuse frequencies towhich high frequency reuse is applied to provide a high traffic carryingcapacity.

At least some of the cells have both at least one regular frequency andat least one super-reuse frequency so that at least one regularfrequency is intended to serve primarily in cell boundary regions and atleast one super-reuse frequency is intended to serve primarily in thevicinity of the base station.

Means controlling or a controller which controls traffic loaddistribution in the cell between at least one regular and at least onesuper-reuse frequency by intra-cell handovers induced by estimatedinterference on said at least one super-reuse frequency.

The invention also relates to a method for increasing traffic carryingcapacity in a cellular radio system. The method includes:

dividing the radio frequencies of the cellular radio network intoregular radio frequencies for which lower frequency reuse is utilized toachieve seamless overall coverage, and super-reuse frequencies to whichhigher frequency reuse is applied to provide a high traffic carryingcapacity,

allocating to at least some of the cells both at least one regularfrequency and at least one super-reuse frequency so that the regularfrequency is intended to serve primarily in cell boundary regions andthe super-reuse frequency is intended to serve primarily in the vicinityof the base station, and

controlling traffic load distribution in the cell between at least oneregular and at least one super-reuse frequency by intra-cell handoversinduced by estimated interference on said at least one super-reusefrequency.

In the invention, the operating frequency spectrum of the cellularnetwork is divided into regular frequencies and super-reuse frequencies.By these two sets of frequencies, two or more separate network layersare provided, at least locally, in the cellular radio network so thattypically both regular frequencies and super-reuse frequencies areemployed in each cell.

One layer, the `overlay layer`, utilizes regular frequencies and aconventional frequency reuse pattern and cell coverage to achieveseamless overall coverage. Frequency planning for regular frequencyreuse is made by conventional criteria using safe handover margins andrequiring low co-channel and adjacent channel interferenceprobabilities. Regular frequencies are intended to serve mobile stationsmainly at cell boundary areas and other locations where the co-channelinterference ratio is poor. The overlay network also providesinterference-free service in the overlapping cell areas required forhandover control and neighboring cell measurements by mobile stations.

Another layer (or several other layers), the `underlay layer`, iscomposed of super-reuse frequencies. The underlay network employs a verytight frequency reuse pattern to provide extended capacity. This isbased on the fact that in the underlay network the same frequency isreused more often than in the overlay network, and hence, moretransceivers can be allocated within the same bandwidth. If thefrequency reuse is, for example, twice as tight as originally, thenumber of transceivers can be doubled. The super-reuse frequencies areintended to serve mobile stations which are close to the BTS, insidebuildings and at other locations where the radio conditions are lessvulnerable to interference.

The cellular network controls traffic division into regular andsuper-reuse frequencies by radio resource allocation at the call set-upphase and later on during the call by handover procedures. The capacityincrease actually provided by such an underlay-overlay network isessentially dependent on how efficiently the mobile stations can bedirected to use the super-reuse frequencies and how well call qualitydeterioration resulting from co-channel interference caused by anincreased level of frequency reuse can be avoided.

In accordance with the invention, this problem is solved by directly anddynamically controlling the co-channel interference level of eachspecific call. The radio network estimates the degree of interference ondifferent frequencies and directs the mobile stations to thosefrequencies that are sufficiently "clean" of interference to sustain agood radio connection quality. More precisely, the cellular networkcontinuously monitors the downlink co-channel interference of eachsuper-reuse frequency in the cell individually for each ongoing call.The call is handed over from a regular frequency to a super-reusefrequency when the co-channel interference level on the super-reusefrequency is sufficiently good. When the co-channel interference levelon the super-reuse frequency deteriorates, the call is handed over fromthe super-reuse frequency back to a regular frequency. Based on theprofile of interference each mobile is exposed to, the cellular networkdetermines the most appropriate frequency for the call connection. Theuse of measured co-channel interference level as a handover decisioncriterion guarantees that the same interference requirements are met forany single cell.

In a first embodiment of the invention, the cell is provided with bothregular and super-reuse frequencies, so that the BCCH frequency of thecell is one of the regular frequencies, whereas the super-reusefrequency is never a BCCH frequency. Call set-up and handover fromanother cell is always first carried out to a regular frequency in thecell, whereafter an underlay-overlay handover in accordance with theinvention to a super-reuse frequency of the cell may be performed.

Stand-alone microcells may be configured solely to use the super-reusefrequencies. Such a microcell is termed a child cell herein. Byestablishing appropriate handover connections, a child cell at a goodlocation, i.e. a traffic hot spot, can handle more traffic than aregular cell in its vicinity. A child cell is an independent cell havinga super-reuse frequency as its BCCH frequency. Traffic is conveyed tothe child cell by means of a handover, since call set-up to the childcell is not possible (only super-reuse frequencies, the interferencelevel cannot be measured prior to call set-up).

In the first embodiment of the invention, the co-channel interferencelevel is estimated by comparing the downlink signal level of the servingcell and the downlink signal levels of those neighbouring cells whichuse the same super-reuse frequencies as the serving cell. The radionetwork can calculate the estimated co-channel interference level at thelocation of each active mobile station. The calculation of theco-channel interference is based on the measurement results on the BCCHfrequencies of the mobile station, which the mobile station measuresalso for a normal handover and reports to the cellular network. In fact,one of the advantages of the invention is that it does not require anymodifications to mobile stations in conventional cellular networks.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in the following by means of preferredembodiments with reference to the accompanying drawing, in which

FIG. 1 illustrates a part of a cellular radio system in which theinvention can be applied,

FIG. 2 illustrates a conventional cellular radio network employing onefrequency reuse pattern,

FIG. 3 illustrates a cellular network in accordance with the invention,employing regular and super-reuse frequencies,

FIG. 4 illustrates underlay and overlay layers provided by thesuper-reuse frequencies and regular frequencies respectively in thenetwork of FIG. 3,

FIG. 5 represents a schematic block diagram of a base station inaccordance with the invention,

FIG. 6 represents a schematic block diagram of a base station controllerin accordance with the invention, and

FIG. 7 illustrates base station-specific and transceiver-specificparameters set in the database of the base station controller.

DETAILED DESCRIPTION OF THE CURRENTLY PREFERRED EMBODIMENT OF THEINVENTION

The present invention can be applied to all cellular radio systems.

The present invention is particularly suited to cellular systemsemploying mobile-assisted cellularly controlled handover, such as thepan-European digital mobile communication system GSM (Global System forMobile Communications) and in other GSM-based systems, such as DCS 1800(Digital Communication System), and in the U.S. digital cellular systemPCS (Personal Communication System). The invention will be described inthe following by using the GSM mobile communication system as anexample. The structure and operation of the GSM system are well known tothose skilled in the art and are defined in the GSM specifications ofETSI (European Telecommunications Standards Institute). Furthermore,reference is made to M. Mouly & M. Pautet, GSM System for MobileCommunication, Palaiseau, France, 1992; ISBN: 2-9507190-0-7.

The general structure of a GSM-type cellular network is illustrated inFIG. 1. The network is composed of two parts: a Base Station SubsystemBSS and a Network Subsystem (NSS). The BSS and mobile stations MScommunicate via radio links. In the base station subsystem BSS, eachcell is served by a base station BTS. A number of base stations areconnected to a base station controller BSC, the function of which is tocontrol the radio frequencies and channels employed by the BTS. Thetasks of the BSC also include handovers in cases where the handover isperformed within the BTS or between two BTSs both of which arecontrolled by the same BSC. The BSCs are connected to a mobile servicesswitching center MSC. Certain MSCs are connected to othertelecommunications networks, such as the public switched telephonenetwork PSTN, and comprise gateway functions for calls to and from suchnetworks. Such MSCs are known as gateway MSCs (GMSC).

For clarity, FIG. 1 shows only one MSC and one base station subsystem inwhich nine base stations BTS1-BTS9 are connected to a base stationcontroller BSC, the radio areas of the base stations constitutingrespective radio cells C1-C9.

1.0 Conventional Cellular Network

The cellular network can be drawn as a combination of circles orhexagons, such as cells C1-C9 in FIG. 1. A hexagonal cell is quitedifferent from the actual world, but is still a good way ofapproximating the network. The cells may have different configurations,such as omni, bisectoral, trisectoral, etc.

The basic principle of a cellular network is frequency reuse, in otherwords, the same frequency is reused in cells with a given spacing. Reuseis usually represented by a frequency reuse pattern, composed of acluster of cells using different frequencies. Cluster size, i.e., thenumber of cells in a cluster, is often used as a yardstick for the reusefactor. For example, in FIG. 2, the reuse pattern or cluster 20comprises 9 cells using mutually different frequencies or sets offrequencies A, B, C, D, E, F, G, H, and I. The same frequencies arereused in clusters of the same size throughout the cellular network. Thecluster size, cell size and spacing between two cells using the samefrequency are determined by the desired C/I (carrier-to-interference)ratio, which is the ratio of the desired receiving signal level to thereceived interference signal. The most significant interference isusually co-channel interference from another cell using the samefrequency. This causes the planning problem stated in connection withthe description of the prior art: Enhancement of frequency reuse, e.g.,reducing cell size, increases the traffic carrying capacity but also theco-channel interference. One prior art solution is a multi-layer networkhaving a "coverage layer" provided by one cell layer, e.g., macrocellnetwork, and a "capacity layer" provided by one cell layer, e.g.,microcell network. However, forming of the other cell layer requiresconsiderable investment and modifications to the network. Furthermore,handover control between the layers has been based on the field strengthor power budget, and thus the connection quality and capacity increaseachieved in the cellular network are questionable, as explained above inconnection with the prior art.

1.1. Conventional Handover

As is well known, mobile stations MS can roam freely within the area ofthe mobile communication system from one cell to another. Handover isonly re-registration with another cell when the mobile station does nothave any ongoing call. When the mobile station MS has an ongoing callduring the handover, also the call must be connected from one basestation to another in a way least disturbing to the call. Transfer of acall from a traffic channel to another traffic channel of the same cellor another cell during a call is termed a handover.

Handover decisions are made by the base station controller BSC on thebasis of the different handover parameters set for each cell and on themeasurement results reported by the mobile station MS and the basestations BTS. Handover is normally induced on the basis of the criteriafor the radio path, but handover may also be due to other reasons,including load distribution. The procedures and calculation on the basisof which the handover decision is made are called a handover algorithm.

For example in accordance with the technical recommendations for the GSMsystem, the mobile station MS monitors (measures) the downlink signallevel and quality of the serving cell and the downlink signal level ofthe cells surrounding the serving cell. The MS is capable of measuring32 cells at most and reporting the measurement results of the six bestBTS to the BSC. The BTS monitors (measures) the uplink signal level andquality, received from each mobile station MS served by said basestation BTS. All measurement results are forwarded to the BSC.Alternatively, all handover decisions can be made in the MSC, in thatcase, the measurement results are forwarded to the MSC as well. The MSCalso controls at least those handovers that are made from the area ofone BSC to the area of another BSC.

When the MS roams in the radio network, handover from the serving cellto a neighboring cell is normally effected either when (1) themeasurement results of the MS and/or BTS show a low signal level and/orsignal quality for the downlink signal of the currently serving cell anda better signal level is to be obtained from a neighboring cell, or when(2) one of the neighboring cells permits communication at lower transmitpower levels, i.e., when the MS is located in the boundary region ofcells. In radio networks, unnecessarily high power levels and therebyinterference to other parts of the network are avoided as far aspossible. The BSC selects, on the basis of the handover algorithmemployed in the system and the reported measurement results, theneighbouring cells whose radio path has properties sufficient for apossible handover. These selected neighboring cells are called handovercandidate cells in this context, and the final target cell for handoveris selected from these. At its simplest, the selection of the targetcell may be effected by selecting a candidate cell having the best radiopath properties, i.e., the best signal level. The candidate cells may,however, be ranked according to specific priority levels on othergrounds as well.

2.0 Underlay-Overlay Network in Accordance With the Invention

In the invention, the operating frequency spectrum of the cellularnetwork is divided into regular frequencies and super-reuse frequencies.By means of these two sets of frequencies, two or more separate "networklayers" are provided, at least locally, in the cellular radio network insuch a way that typically both regular frequencies and super-reusefrequencies, to which mutually different reuse factors are applied, areemployed in each cell. An exception is made by a child cell, which willbe described in detail below. FIG. 3 illustrates a cellular network inaccordance with the invention, which has been formed by addingsuper-reuse frequencies into the cells of the conventional cellularnetwork of FIG. 2. FIG. 4 illustrates how the regular and super-reusefrequencies in the cells form two separate "network layers" in thefrequency domain.

One layer 41, the `overlay layer`, utilizes the regular frequencies ofcells 10, i.e., A, B, C, D, E, G, H, and I, and a conventional frequencyre-use pattern and cell radius to produce seamless overall coverage.Frequency planning for regular frequency reuse is made by conventionalcriteria using safe handover margins and requiring low co-channel andadjacent channel interference probabilities. Regular frequencies areintended to serve mobile stations mainly at cell boundary areas andother locations where the co-channel interference ratio is poor. Theoverlay network also provides interference-free service in theoverlapping cell areas required for handover control and neighboringcell measurements by a mobile station. Hence, an overlay network istypically a conventional cellular radio network. It may also be a celllayer in a network comprising two physical cell layers, e.g., amacrocell, microcell or picocell layer. In such a case, the frequencyspectrum division in accordance with the invention is carried out withinthe physical macrocells, microcells or picocells. In the example ofFIGS. 3 and 4, the overlay network is a unidimensional cellular networkin accordance with FIG. 2 in which the cluster size is 8.

Another layer (or several other layers) 42, the `underlay layer`, iscomposed of the super-reuse frequencies S1, S2, and S3 of the cells. Itis thus to be noted that the invention typically employs only onephysical cell layer and that the overlay and underlay layers are notmade up by different physical cells by different frequencies or sets offrequencies in the same physical cell. The underlay network employs avery tight frequency reuse pattern, so that a smaller coverage,represented by a small hexagon 11 in FIG. 3, is created for thesuper-reuse frequencies within regular cell 10. Provision of extendedcapacity is based on the fact that in the underlay network the samefrequency can be reused more often than in the overlay network, andhence more transceivers can be allocated within the same bandwidth. Inthe example of FIGS. 3 and 4, the cluster size of the underlay layer is3, and thus the number of transceivers per frequency can be nearlytripled compared with the overlay layer (cluster size 8): Thesuper-reuse frequencies are intended to serve mobile stations which areclose to the BTS, inside buildings and at other locations where theradio conditions are less vulnerable to interference. As illustrated inFIGS. 3 and 4, the super-reuse frequencies do not provide continuouscoverage but rather form separate islands. However, it is possible,depending on the frequency reuse factor, that also the super-reusefrequency coverages overlap.

A cellular network may employ several sets of super-reuse frequencies towhich similar or different reuse is applied independently. Each set ofsuper-reuse frequencies thus forms a distinct underlay "network layer".

Division of cell frequencies into regular and super-reuse frequencies iscontrolled transceiver-specifically at the base station BTS. All radiotransceivers (TRX) at the BTS are defined either as regular TRXs orsuper-reuse TRXs (a child cell is an exception). The radio frequency ofa regular TRX is among the regular frequencies, i.e., A-I. The radiofrequency of a super-reuse TRX is one of the super-reuse frequencies,i.e., S1, S2 and S3. Each BTS must additionally have a `broadcastcontrol channel frequency` (BCCH frequency) which is measured by the MSin adjacent cell measurements, for example. In a first embodiment of theinvention, the BCCH frequency is always one of the regular frequencies.A child cell again makes an exception; therein the BCCH frequency is asuper-reuse frequency.

The base station BTS of cell 10 is typically furnished with both typesof TRX. FIG. 5 illustrates a BTS in accordance with the invention,comprising two TRXs 51 and 52. It is to be noted, however, that theremay be any desired number of TRXs. TRX 51 is a regular TRX whose radiofrequency A is one of the regular frequencies and also provides the BCCHfrequency for the cell. TRX 52 is a super-reuse TRX whose radiofrequency S1 is one of the super-reuse frequencies. TRXs 51 and 52 areconnected via a combiner and divider unit 54 to common transmitting andreceiving antennas ANT_(TX) and ANT_(RX). Regular and super-reusefrequencies may also have separate antennas, for instance to obtain asadvantageous a coverage as possible for the super-reuse frequencies.TRXs 51 and 52 are further connected to transmission system equipment 55providing connection to a transmission link to the BSC, e.g., a 2 Mbit/sPCM link. The operation of the TRXs 51 and 52 is controlled bycontroller unit 53 having a signalling connection with the BSC throughthe transmission system equipment 55. The BTS in accordance with theinvention may be a fully commercial base station, e.g., GSM Base StationDE21 by Nokia Telecommunications Oy. What is essential to the inventionis the division of the frequencies used by the TRXs.

An exception to the cell and base station principle presented above is a`child cell`. A child cell is an individual physical microcell having asuitable location, i.e., a traffic hot spot, and configured to usesuper-reuse frequencies only. In other words, in view of frequencyspectrum division, the child cell is located on one of the underlaylayers and is capable of handling more traffic than a regular cell inits vicinity by establishing appropriate handover connections. Since achild cell is an independent cell, it employs a super-reuse frequency asits BCCH frequency. For a child cell, however, in the primary embodimentof the invention a barring parameter preventing call set-up directly tothe child cell is sent on the BCCH frequency. Thus, a child cell canonly be reached by handover from an adjacent regular cell, which istermed a parent cell. FIG. 4 shows a child cell 44 having a super-reusefrequency S4.

The cellular network, in the primary embodiment of the invention a BSC,controls traffic division into regular and super-reuse frequencies bymeans of radio resource allocation at the call set-up phase and later onduring the call by means of handover procedures. FIG. 6 shows aschematic block diagram of a BSC. A group switch GSW 61 provides theconnection operation of the BSC. Besides routing calls between the basestations BTS and the MSC, the group switch GSW is employed to connectcalls in intra-BSC handovers. Controller unit 62 handles all controloperations within the base station subsystem BSS, such as the executionof handover algorithms. Network configuration database 63 contains allhandover and configuration parameters of the base station subsystem BSS.All parameters required by the underlay-overlay feature in accordancewith the invention are stored in the database 63. One of the basestation-specific and TRX-specific parameter settings included in thedatabase 63 is depicted in FIG. 7; herein TRX-specific parametersdefine, for instance, whether a regular or a super-reuse TRX isconcerned. The other parameters will be described in detail below. Thepresent invention only requires the modifications to be more closelydescribed below to the functions of the controller unit 62 and to theparameter settings in the database 63. Otherwise the BSC of theinvention can be implemented with any commercial BSC.

3.0 Intelligent Underlay-Overlay Handover

3.1 General Principle

The capacity increase practically provided by the underlay-overlaynetwork of the invention is dependent on how efficiently the mobilestations MS can be directed to use the super-reuse frequencies and howwell call quality deterioration is simultaneously avoided.

In the invention, the BSC controls traffic division into regular andsuper-reuse frequencies by means of radio resource allocation at thecall set-up phase and later on during the call by means of a handover.The BSC allocates a traffic channel to the call to be set up or to acall handed over from another regular cell at a regular TRX only,wherefore a regular cell must have at least one regular TRX, typically aBCCH TRX, as illustrated in FIG. 5. After this, the BSC monitors thedownlink C/I ratio on each super-reuse frequency of the regular cellseparately for each ongoing call. The monitoring is accomplished in sucha way that the BSC calculates the downlink C/I ratio of the super-reuseTRX by means of various parameters and by means of measurement resultsreported by the MS via the BTS. The principle of the C/I evaluation issimple. By comparing the downlink signal level of the serving cell(C=Carrier) and the downlink signal levels of the neighbouring cells(I=Interference) which use the same super-reuse frequencies as theserving cell, the BSC can calculate the C/I ratio on the super-reusefrequencies at the location of each active mobile station MS. The C/Ican be calculated in this way, since the downlink transmit power is thesame on the regular and super-reuse frequencies of the cell.

Example: A super-reuse frequency 90 has been allocated to cells A and B,and the cells are close enough to each other to cause interference. Whenthe downlink signal level of the serving cell A is -70 dBm and thesignal level of the adjacent cell B is -86 dBm, the downlink C/I ratioof the super-reuse TRX (frequency 90) of cell A is 16 dB.

The BSC always hands the call from the regular TRX over to thesuper-reuse TRX when the downlink C/I ratio of the super-reuse TRX issufficiently good (handover HO₂ in FIG. 4). If the downlink C/I ratio ofthe super-reuse TRX becomes poor, the BSC again hands the call from thesuper-reuse TRX over to the regular TRX in the same cell (handover HO₃in FIG. 4). If there is also a child cell under the BSC--such as childcell 44 in FIG. 4--which is adjacent to the regular/serving cell, theBSC continuously monitors the downlink C/I ratio of each super-reusefrequency of the child cell during each call. The call is handed overfrom the regular cell to the child cell when the downlink C/I ratio ofthe child cell is sufficiently good (handover HO₅ in FIG. 4). If thedownlink C/I ratio of the child cell becomes poor, the call is handedover from the child cell to one regular/parent cell adjacent to thechild cell (handover HO₆ in FIG. 4).

The above-described radio resource allocation and handovers togetherform an intelligent underlay-overlay feature in the cellular network;this feature is controlled by means of various parameters as illustratedin FIG. 7. These required parameters are stored in the networkconfiguration database 63 at the BSC (FIG. 6). The network operator canadminister the parameters for example through the operations andmaintenance centre OMC of the network. The underlay-overlay feature inaccordance with the invention has special requirements for every stageof the handover algorithm: processing of measurement results, thresholdcomparison and decision algorithm. Nevertheless, the intelligentunderlay-overlay feature in accordance with the invention is stillcompatible with the above-described standard handover algorithm. This isdue to the fact that the BSC uses different handover decision algorithmsfor handovers arising from traffic control between regular andsuper-reuse frequencies than for handovers arising from conventionalradio path criteria, such as power budget, low signal level or poorsignal quality.

In the following, the main steps of the underlay-overlay handover inaccordance with the invention are described in detail, these stepsbeing: 1) processing of radio link measurements, 2) C/I determinationprocedure, 3) handover threshold comparison, and 4) selection of ahandover candidate.

3.2 Processing of Radio Link Measurements

As stated previously, underlay-overlay handover decisions made by theBSC are based on measurement results reported by the MS and on variousparameters. Database 63 at the BSC is capable of maintaining ameasurement table of 32 neighboring cells per each call and storing themeasurement results as they arrive. Furthermore, a specific number ofinterfering cells has been defined for each super-reuse TRX, asillustrated in FIG. 7. The interfering cells must be adjacent to theserving cell, as the MS only measures cells defined in the list ofneighboring cells. In the first embodiment of the invention, for the BSCto be able to monitor several super-reuse TRXs and cells interferingwith them simultaneously for one call, it must be possible to definefive interfering cells at most to each super-reuse TRX. This enablessimultaneous monitoring of all super-reuse TRXs at the BSC.

The information on the cells being measured is sent to the MS in aneighboring cell list. The MS measures the cells defined in the list andreports the measurement results of the six strongest neighboring cellsto the BSC. The interfering cells must be adjacent to the serving cell,otherwise the MS is not capable of measuring and reporting the signallevels of the interfering cells. In any case, the measurement results ofthe interfering cells are often weaker than those of the six strongestneighboring cells, wherefore the measured downlink level RXLEV of theinterfering cell is available only intermittently.

When the RXLEV of the interfering cell is missing from the measurementresults, the steps to be taken vary depending on whether the RXLEV ofthe interfering cell is considered as a directly measured interferencelevel or whether the RXLEV of the interfering cell is a reference valuewhich is used for calculating an interference level estimate, as will beexplained in item 3.3.

1) Directly measured interference level. When the MS reports themeasurement results of the six neighboring cells, that is, the sixpositions in the measurement sample are occupied, the weakest RXLEV ofthe six reported cells is entered as the measurement value for thoseinterfering cells that are missing from the measurement sample. When theMS reports the measurement results of less than six neighbouring cells,a zero is entered as the measurement value for those interfering cellsthat are missing from the measurement sample.

2) The measurement value is used for calculating an interference levelestimate. For those cells whose measurement values are used forcalculating an interference level estimate and are missing from themeasurement sample, a zero is entered as the measurement value.

In order for the result to have maximum reliability, the BSC cancalculate an average of several measurement results, which is then usedin the C/I evaluation.

3.3. C/I Evaluation

C/I evaluation is carried out each time the BSC receives measurementresults and an average of these is calculated.

If the call is on a regular TRX, the C/I evaluation concerns everysuper-reuse TRX of the serving cell and those child cells which areadjacent to the serving cell. In such a case, the evaluation strives tofind a super-reuse TRX having a sufficiently good C/I ratio forhandover.

If the call has been handed over to a super-reuse TRX, the C/Ievaluation concerns only the super-reuse TRX itself. In such a case, thepurpose of the evaluation is to monitor whether the C/I ratio of thesuper-reuse frequency is good enough or whether the call is to be handedover to a regular frequency.

The BSC calculates the downlink C/I ratio of the super-reuse TRX in themanner set out above by the processed measurement results (averages) andthe parameters set for said TRX. The processed measurement results arethe downlink RXLEV of the serving cell, the downlink RXLEV of theinterfering cells and the downlink RXLEV of the child cell. Theparameters are Level Adjustment, CIEstWeight and CIEstType; these areset for the TRX in database BSC (FIG. 7). Level Adjustment is theadjustment level of the interfering cell (-63 dB . . . 63 dB), which isused to calculate an interference level estimate from the signal levelof the interfering cell. CIEstWeight is the weighting coefficient of theinterfering cell (1 . . . 10). CIEstType indicates whether the signallevel of the interfering cell is considered as a directly measuredinterference level or whether the signal level of the interfering cellis a reference value which is used for calculating an interference levelestimate.

By comparing the downlink RXLEV of the super-reuse TRX and the downlinkinterference level, the BSC can calculate the C/I ratio of thesuper-reuse TRX.

3.3.1. Calculation of the RXLEV of the Super-Reuse TRX

For the above comparison, the RXLEV of the super-reuse TRX must first bedetermined.

In the following, cases in which the super-reuse TRX is allocated to aregular cell (case 1) or to a child cell (cases 2 and 3) are considered.

1) The average downlink receiving level AV₋₋ RXLEV₋₋ TRX(k) of thesuper-reuse TRX of a regular cell is calculated in the following way:

    (AV.sub.-- RXLEV-TRX(k)=AV.sub.-- RXLEV.sub.-- DL.sub.-- HO+(BS-TXPWR.sub.-- MAX-BS.sub.-- TXPWR)                  (1)

where AV₋₋ RXLEV₋₋ DL₋₋ HO is the average downlink RXLEV of the servingcell. BS₋₋ TXBWR₋₋ MAX-BS₋₋ TXBWR is the difference between the maximumdownlink RF power permitted in the serving cell and the actual downlinkpower due to the BTS power control.

2) When the child cell is the handover candidate, the average downlinkreceiving level AV₋₋ RXLEV₋₋ TRX(k) of the super-reuse TRX equals theaverage downlink receiving level of the child cell.

3) When the child cell is the serving cell, the average downlinkreceiving level AV₋₋ RXLEV₋₋ TRX(k) of the super-reuse TRX is calculatedin the following way:

    AV.sub.-- RXLEV.sub.-- TRX(k)=AV.sub.-- RXLEV.sub.-- DL.sub.-- HO+(BS.sub.-- TXPWR.sub.-- MAX-BS.sub.-- TXPWR)           (2)

3.3.2. Directly Measured Interference Level

The most common situation is that the interfering cell is a regular cellwhich is adjacent to the serving cell and the interfering cell has thesame set of super-reuse frequencies as the serving cell. Also thelocation of the interfering cell is close enough to cause interference.In this situation, the average downlink receiving level AV₋₋ RXLEV₋₋INFx(k) of the interfering cell corresponds directly to the interferencelevel I on the super-reuse TRX caused by the interfering cell.

3.3.4. Estimated Interference Level

If the call is on a super-reuse TRX (BCCH frequency) of the child cellor the child cell is a handover candidate and the potential source ofinterference is another child cell with the same super-reuse frequency(also BCCH frequency), the corresponding interference level cannot bedirectly measured and reported by the MS because of the same BCCHfrequencies. In this case, the BSC can only estimate the level ofinterference caused by the other child cell by means of the signallevels which the MS can measure and report.

If the RF signal profile of a regular adjacent cell is similar to theinterference profile within the coverage area of the serving cell, it ispossible to define the regular adjacent cell as the interfering cell(reference cell) instead of the true source of interference. The RFsignal profile is considered the same as the interference profile whenthe ratio between the RF signal level and the interference level (forexample 6 dB) remains approximately unchanged within the service area ofthe serving cell. This ratio is represented by means of the aboveparameter LevelAdjustment set for each interfering or reference cell, asillustrated in FIG. 7. The type of the adjacent cell is indicated bymeans of the parameter CIEstType.

3.3.3.1 Interference Level Estimation Based on Several Cells

In order to increase the reliability of the estimation, severalreference cells may be used for calculating the estimated downlinkinterference level AV₋₋ RXLEV₋₋ ESTM(k). AV₋₋ RXLEV₋₋ ESTM(k) and thedownlink C/I ratio of the super-reuse TRX are calculated by usingsimilar evaluation methods. Various mathematical methods, such as theaverage taking method and the maximum taking method, can be used tocalculate the downlink C/I ratio of the super-reuse TRX or the estimateddownlink interference level. A cellular network may employ severalcalculation methods, which are selected for instance cell-specificallyby means of special parameters. The average taking method will bedescribed by way of example in the following.

Average Taking Method

The estimated downlink interference level AV₋₋ RXLEV₋₋ ESTM(k) iscalculated by means of the average taking method in the following way(when only the RXLEVs of the reference cells are taken into account):##EQU1##

The downlink C/I ratio CI₋₋ RATIO(k) of the super-reuse transceiverTRX(k) is calculated by the average taking method in the following way(when only the RXLEVs of the interfering cells are taken into account;instead of the RXLEVs of the reference cells, the downlink interferencelevel AV₋₋ RXLEV₋₋ ESTM(k) estimated by means of equation 3 is used):##EQU2##

LEV₋₋ ADJ₋₋ INFTx(k) is the adjustment parameter (LevelAdjustment) ofthe interfering/reference cell and Vx(k) is the weighting coefficient ofthe interfering/reference cell (parameter CIEstWeight, set for eachinterfering cell).

3.4. Handover Threshold Comparison

The underlay-overlay feature in accordance with the invention introducestwo special handover thresholds in addition to the normal handoverthresholds:

SuperReuseGoodCiThreshold is a threshold used in the comparison of thedownlink C/I ratio of the super-reuse TRX to initiate a handover to thesuper-reuse TRX.

SuperReuseBadCiThreshold is a threshold used in the comparison of thedownlink C/I ratio of the super-reuse TRX to initiate a handover awayfrom the super-reuse TRX. Both handover thresholds are composed of threeparts: the actual threshold (CiRatio), the total number of comparisons(Nx) to be taken into account before a decision is possible, the numberof comparisons out of total comparisons (Px) where the downlink C/Iratio has to be lower/greater than or equal to the threshold before anymeasures are possible. Each time the PSC receives measurement resultsfrom MS1 (e.g. after each SACCH multiframe), the BSC compares thedownlink C/I ratio of specified super-reuse TRXs with a specifiedhandover threshold. When the call is on a regular TRX, the thresholdcomparison concerns every super-reuse TRX of the serving cell and thosechild cells which are adjacent to the serving cell, and the handoverthreshold is SuperReuseGoodCiThreshold. If the call has been handed overto a super-reuse TRX, the threshold comparison concerns only thesuper-reuse TRX itself and the handover threshold isSuperReuseBadCiThreshold.

The threshold comparison and the steps to be taken are as follows:

1) Comparison of the downlink C/I ratio CI₋₋ RATIO(k) withSuperReuseGoodCiThreshold. If at least in Bx comparison out of the totalNx comparisons the downlink C/I ratio of the super-reuse TRX, CI₋₋RATIO(k), is greater than or equal to the threshold CiRatio, a handoverfrom a regular TRX to a super-reuse TRX(k) can be made on account of thegood C/I ratio.

2) Comparison of the downlink C/I ratio CI₋₋ RATIO(k) withSuperReuseBadCiThreshold. If at least in Bx comparison out of the totalNx comparisons the downlink C/I ratio of the super-reuse TRX, CI₋₋RATIO(k), is lower than or equal to the threshold CiRatio, a handoverfrom a super-reuse TRX(k) to a regular TRX is required on account of thebad C/I ratio.

3.5. Handover Decision Algorithms

3.5.1. Intra-cell Handover From a Regular TRX to a Super-reuse TRX

The BSC recognises the possibility to make a handover when the handoverthreshold comparison indicates that a handover, the cause of which is agood C/I ratio, can be made from a regular TRX to a specifiedsuper-reuse TRX. If there are several super-reuse TRXs in the servingcell which meet the handover requirements for the C/I ratiosimultaneously, the handover algorithm ranks the super-reuse TRXsaccording to the C/I ratios. If there is an appropriate super-reuse TRXin the serving cell and in the child cell at the same time, the BSCprefers the child cell to the serving cell. In other words, the BSCperforms an inter-cell handover to the child cell instead of theintra-cell handover.

3.5.2. Intra-cell Handover From a Super-reuse TRX to a Regular TRX

The BSC recognises the necessity to make a handover when the handoverthreshold comparison indicates that some of the following criteria for ahandover are present: downlink interference, downlink quality and badC/I ratio. When the cause of the handover attempt is downlinkinterference or downlink quality and the intra-cell handover to aregular TRX fails, the BSC may perform a handover to another regularcell in order to maintain the call.

3.5.3. Intra-cell Handover Between Super-reuse TRXs

The BSC recognises the necessity to make a handover when the handoverthreshold comparison indicates than an intra-cell handover, the cause ofwhich is uplink interference, might be required. If the intra-cellhandover attempt to another super-reuse TRX fails or the handover is notenabled, the BSC may perform either an intra-cell handover or aninter-cell handover to a regular TRX in order to maintain the call.

3.5.4. Inter-cell Handover From a Regular Cell to a Child Cell

The BSC recognizes the possibility to make a handover when the handoverthreshold comparison indicates that a handover, the cause of which is agood C/I ratio, could be made from a regular TRX to a specifiedsuper-reuse TRX of the child cell. In order for the handover to thechild cell to become possible, the child cell must also satisfy thefollowing requirements for the radio link properties:

    1. AV.sub.-- RXLEV.sub.-- NCELL(n)>RXLEV.sub.-- MIN(n)+MAX(0, Pa) where Pa=(MS.sub.-- TXPWR.sub.-- MAX(n)-P)                      (5)

    2. PBGT(n)>HO.sub.-- MARGIN.sub.-- PBGT(n)

RXLEV₋₋ MIN(n) is the level which the signal level AV₋₋ RXLEV₋₋ NCELL(n)in the child cell (n) must exceed before the handover is possible. Thisparameter is set for each adjacent cell for the normal handoveralgorithm. MS₋₋ TXBWR₋₋ MAX(n) is the maximum RX power than an MS ispermitted to use on a traffic channel in the adjacent cell. H₋₋ MARGIN₋₋BGT(n) is the margin which the power budget PBGT(n) of the child cell(n) must exceed before the handover is possible. Also these areparameters that are set for each adjacent cell for the normal handover.B is the maximum power of the MS.

If there are appropriate super-reuse frequencies in many child cells,the BSC ranks the child cells according to priority levels and the loadof the child cells and selects the best child cell to be the targetcell. If there are several super-reuse TRXs in the child cell which meetthe requirements for the C/I ratio simultaneously, the handoveralgorithm ranks the TRXs according to the C/I ratios.

3.5.5. Inter-cell Handover From a Child Cell to a Regular Cell

The BSC recognises the necessity to make a handover when the handoverthreshold comparison indicates that some of the following criteria for ahandover are present: downlink interference, downlink quality and badC/I ratio. If there are several regular cells available, the BSC selectsone regular cell which has the best signal strength condition to be thetarget cell. If there are no regular cells available within the area ofthe BSC, the BSC may initiate an inter-BSC handover caused by theconventional tradition criteria in order to maintain the call. Aftercall set-up and after all handovers, there is preferably a given periodof time during which the C/I evaluation is considered unreliable and thehandover is not allowed. This period is allowed for the MS to decode theidentifiers BSIC of the interfering/reference cells before the C/Ievaluation is started. Furthermore, repeated handovers for the same MSare preferably prevented by setting a minimum interval between handoversrelated to the same connection. Furthermore, if a handover attempt failsfor some reason, a new attempt to the same connection is only permittedafter a minimum interval.

The figures and the description pertaining to them are only intended toillustrate the present invention. In its details, the present inventionmay vary within the scope and spirit of the attached claims.

We claim:
 1. A cellular radio network including allocated radiofrequencies reused in cells, comprising:said allocated radio frequenciesbeing divided into regular radio frequencies for which lower frequencyreuse is utilized to achieve a seamless overall coverage, andsuper-reuse frequencies to which high frequency reuse is applied toprovide a high traffic carrying capacity; at least some of said cellshaving both at least one regular frequency and at least one super-reusefrequency, so that said at least one regular frequency is intended toserve primary in cell boundary regions and said at least one super-reusefrequency is intended to serve primary in the vicinity of a basestation; and a controller which controls traffic load distribution in acell between said at least one regular and said at least one super-reusefrequency by intra-cell handovers induced by estimated interference onsaid at least one super-reuse frequency, wherein a BCCH frequency of thecell is a regular frequency, and wherein a radio frequency assigned incall-setup and handover from another cell is always a regular frequency.2. The cellular radio network as claimed in claim 1,wherein a handoverfrom a regular frequency to a super-reuse frequency occurs at apredetermined interference level on said super-reuse frequency, andwherein a handover from a super-reuse frequency to a regular frequencyoccurs when there is too poor an interference level on said super-reusefrequency.
 3. The cellular radio network as claimed in claim 1, furthercomprising:at least one microcell having only super-reuse frequencies,one of said super-reuse frequencies being a BCCH frequency, and callset-up in a microcell is barred, and said controller controls trafficload distribution between regular cells and said microcell by inter-cellhandovers induced by an interference level in said microcell.
 4. Thecellular radio network as claimed in claim 1, further comprising:amobile-assisted handover procedure in which a mobile station measures asignal receiving level of a serving cell and a signal level of adjacentcells and forwards said measurement results to said handover controllerof said cellular network, wherein said handover controller estimates aninterference level on said super-reuse frequencies of said serving cellbased on said measurement results.
 5. The cellular radio network asclaimed in claim 4, wherein one or more adjacent cells have beenassigned to each super-reuse frequency of said serving cell, saidmeasured receiving level of said adjacent cell being used to estimateinterference on said super-reuse frequency.
 6. The cellular radionetwork as claimed in claim 4, wherein said measurement results of saidmobile station only concern a limited number of ambient cells, and thatat least one reference cell has been assigned to at least onesuper-reuse frequency of said serving cell from among said ambientcells, said reference cell having an interference profile of a typesimilar to an interference profile of a more remote cell which is apotential source of interference on said super-reuse frequency butcannot be directly measured by said mobile station, and that saidhandover controller estimates said interference level caused by saidmore remote cell on said super-reuse frequency, using said measuredsignal level of said reference cell.
 7. The cellular radio network asclaimed in claim 6, wherein a handover algorithm is adapted to estimatea signal level of an interfering cell by correcting said measuredreceiving level of said reference cell taking into account a differencein signal levels of said reference cell and an actual interfering cell.8. A method for increasing traffic carrying capacity in a cellular radiosystem, comprising:dividing radio frequencies of said cellular radionetwork into regular radio frequencies for which lower frequency reuseis utilized to achieve seamless overall coverage, and super-reusefrequencies to which higher frequency reuse is applied to provide a hightraffic carrying capacity; allocating to at least some cells of saidcellular radio network both at least one regular frequency and at leastone super-reuse frequency so that said regular frequency is intended toserve primarily in cell boundary regions and said super-reuse frequencyis intended to serve in a vicinity of a base station; controllingtraffic load distribution in a cell between said at least one regularand said at least one super-reuse frequency by intra-cell handoversinduced by estimated interference on said at least one super-reusefrequency; allocating a regular frequency as a BCCH frequency of saidcell in each case; and always assigning a regular frequency in callset-up and in a handover from another cell in each case.
 9. The methodas claimed in claim 8, further comprising:performing an intra-cellhandover from a regular frequency to a super-reuse frequency when saidsuper-reuse frequency has a predetermined interference level; andperforming a handover from a super-reuse frequency to a regularfrequency when said super-reuse frequency has too poor an interferencelevel.
 10. The method as claimed in claim 8, furthercomprising:measuring a signal receiving level and quality of a servingcell at said mobile station; measuring said signal receiving level ofcells ambient to said serving cell at said mobile station; forwardingmeasurement results from said mobile station to said cellular radionetwork; and estimating an interference level on said super-reusefrequencies of said serving cell based on said measurement results. 11.The method as claimed in claim 10, further comprising:assigning one ormore adjacent cells to each super-reuse frequency of said serving cell,said measured receiving level of the adjacent cell being used toestimate said interference level on said super-reuse frequency.
 12. Themethod as claimed in claim 10,wherein said measurement results reportedby said mobile station only concern a limited number of ambient cells,said method further comprising:assigning at least one reference cell toat least one super-reuse frequency of said serving cell from among saidambient cells, said reference cell having an interference profile of atype similar to an interference profile of a more remote cell which is apotential source of interference on said super-reuse frequency butcannot be directly measured by said mobile station; and estimating aninterference level caused by said more remote cell on said super-reusefrequency using said measured signal level of said reference cell. 13.The method as claimed in claim 12, further comprising:correcting saidmeasured signal level of said reference cell taking into account adifference in signal levels of said reference cell and said remote cellin estimating said interference level.
 14. A cellular radio networkincluding allocated radio frequencies reused in cells, comprising:saidallocated radio frequencies being divided into regular radio frequenciesfor which lower frequency reuse is utilized to achieve a seamlessoverall coverage, and super-reuse frequencies to which high frequencyreuse is applied to provide a high traffic carrying capacity; at leastsome of said cells having both at least one regular frequency and atleast one super-reuse frequency, so that said at least one regularfrequency is intended to serve primary in cell boundary regions and saidat least one super-reuse frequency is intended to serve primary in thevicinity of a base station; and a controller which controls traffic loaddistribution in a cell between said at least one regular and said atleast one super-reuse frequency by intra-cell handovers induced byestimated interference on said at least one super-reuse frequency,wherein a BCCH frequency of the cell is a regular frequency, and whereina radio frequency assigned in call-setup is always a regular frequency.15. The cellular radio network as claimed in claim 14, wherein a radiofrequency assigned in a handover from another cell is always a regularfrequency.
 16. The cellular radio network as claimed in claim 14,wherein a handover from a regular frequency to a super-reuse frequencyoccurs at a predetermined interference level on said super-reusefrequency, andwherein a handover from a super-reuse frequency to aregular frequency occurs when there is too poor an interference level onsaid super-reuse frequency.
 17. The cellular radio network as claimed inclaim 14, further comprising:at least one microcell having onlysuper-reuse frequencies, one of said super-reuse frequencies being aBCCH frequency, and call set-up in a microcell is barred, and saidcontroller controls traffic load distribution between regular cells andsaid microcell by inter-cell handovers induced by an interference levelin said microcell.
 18. The cellular radio network as claimed in claim14, further comprising:a mobile-assisted handover procedure in which amobile station measures a signal receiving level of a serving cell and asignal level of adjacent cells and forwards said measurement results tosaid handover controller of said cellular network, wherein said handovercontroller estimates an interference level on said super-reusefrequencies of said serving cell based on said measurement results. 19.The cellular radio network as claimed in claim 18, wherein one or moreadjacent cells have been assigned to each super-reuse frequency of saidserving cell, said measured receiving level of said adjacent cell beingused to estimate interference on said super-reuse frequency.
 20. Thecellular radio network as claimed in claim 18, wherein said measurementresults of said mobile station only concern a limited number of ambientcells, and that at least one reference cell has been assigned to atleast one super-reuse frequency of said serving cell from among saidambient cells, said reference cell having an interference profile of atype similar to an interference profile of a more remote cell which is apotential source of interference on said super-reuse frequency butcannot be directly measured by said mobile station, and that saidhandover controller estimates said interference level caused by saidmore remote cell on said super-reuse frequency, using said measuredsignal level of said reference cell.
 21. The cellular radio network asclaimed in claim 20, wherein a handover algorithm is adapted to estimatea signal level of an interfering cell by correcting said measuredreceiving level of said reference cell taking into account a differencein signal levels of said reference cell and an actual interfering cell.22. A method for increasing traffic carrying capacity in a cellularradio system, comprising:dividing radio frequencies of said cellularradio network into regular radio frequencies for which lower frequencyreuse is utilized to achieve seamless overall coverage, and super-reusefrequencies to which higher frequency reuse is applied to provide a hightraffic carrying capacity; allocating to at least some cells of saidcellular radio network both at least one regular frequency and at leastone super-reuse frequency so that said regular frequency is intended toserve primarily in cell boundary regions and said super-reuse frequencyis intended to serve in a vicinity of a base station; controllingtraffic load distribution in a cell between said at least one regularand said at least one super-reuse frequency by intra-cell handoversinduced by estimated interference on said at least one super-reusefrequency; allocating a regular frequency as a BCCH frequency of saidcell in each case; and always assigning a regular frequency in callset-up in each case.
 23. The method as claimed in claim 22, whereinalways assigning a regular frequency in a handover from another cell ineach case.
 24. The method as claimed in claim 22, furthercomprising:performing an intra-cell handover from a regular frequency toa super-reuse frequency when said super-reuse frequency has apredetermined interference level; and performing a handover from asuper-reuse frequency to a regular frequency when said super-reusefrequency has too poor an interference level.
 25. The method as claimedin claim 22, further comprising:measuring a signal receiving level andquality of a serving cell at said mobile station; measuring said signalreceiving level of cells ambient to said serving cell at said mobilestation; forwarding measurement results from said mobile station to saidcellular radio network; and estimating an interference level on saidsuper-reuse frequencies of said serving cell based on said measurementresults.
 26. The method as claimed in claim 25, furthercomprising:assigning one or more adjacent cells to each super-reusefrequency of said serving cell, said measured receiving level of theadjacent cell being used to estimate said interference level on saidsuper-reuse frequency.
 27. The method as claimed in claim 25, whereinsaid measurement results reported by said mobile station only concern alimited number of ambient cells, said method furthercomprising:assigning at least one reference cell to at least onesuper-reuse frequency of said serving cell from among said ambientcells, said reference cell having an interference profile of a typesimilar to an interference profile of a more remote cell which is apotential source of interference on said super-reuse frequency butcannot be directly measured by said mobile station; and estimating aninterference level caused by said more remote cell on said super-reusefrequency using said measured signal level of said reference cell. 28.The method as claimed in claim 27, further comprising:correcting saidmeasured signal level of said reference cell taking into account adifference in signal levels of said reference cell and said remote cellin estimating said interference level.